The process of electrophotography as disclosed by Carlson in U.S. Pat. No. 2,297,691, employs an electrophotographic element comprising a support material bearing a coating of an insulating material whose electrical resistance varies with the amount of incident electromagnetic radiation it receives during an imagewise exposure. The element, commonly termed an electrophotographic element, is first given a uniform surface charge in the dark after a suitable period of dark adaptation. It is then exposed to a pattern of actinic radiation which has the effect of differentially reducing the potential of this surface charge in accordance with the relative energy contained in various parts of the radiation pattern.
The differential surface charge, or electrostatic latent image, remaining on the electrophotographic element is then developed by contacting the surface with a suitable electroscopic marking material. Such marking material, or toner, whether contained in an insulating liquid or in a dry developer, is deposited on the exposed surface in accordance with either the charge pattern or discharge pattern depending on the charge polarity of the toner and the surface of the element. Deposited marking material is either permanently fixed to the surface of the electrophotographic element by means such as heat, pressure, or solvent vapor, or transferred to a receiver element to which it is similarly fixed. Likewise, the electrostatic charge pattern can be transferred to a receiver element and developed there.
There are a variety of different configurations for electrophotographic elements. An electrophotographic element may comprise a homogeneous photoconductive layer, it may comprise an aggregate layer containing a photoconductor and a sensitizing dye, or it may be a composite or multilayer element.
An example of an electrophotographic element comprising a single homogeneous photoconductive layer is one having a film-forming polymeric organic photoconductor and sensitizing dye coated on an electrically conductive substrate. In such an element the sensitizing dye and the organic photoconductor are dissolved uniformly through the photoconductive layer and no heterogeneity can be seen under high magnification.
Electrophotographic elements comprising aggregate layers typically comprise an electrically conductive substrate, which is coated with sensitizing dye dispersed in a polymeric binder. In these elements the dye and some of the polymer combine (aggregate) together to form a crystal-like complex which is visible under magnification and is randomly distributed through the photoconductive layer.
Multilayer or composite electrophotographic elements typically comprise three layers. The first being an electrically conductive substrate coated with a charge-generation layer upon which is coated a charge-transport layer. Generally in elements of this type the charge-transport layer, containing no sensitizer (i.e. no charge-generation material) is homogeneous under high magnification. The charge-generation layer is coated as a thin separate layer underneath the charge-transport layer. Charge-transport material is often added to this charge-generation layer. Next, in turn, is the conductive layer. Examples of these three types of electrophotographic elements are well known in the art.
U.S. Pat. No. 4,140,529 discloses a photoconductive element having a charge-transport overlayer. The charge-transport layer comprises an organic resinous material comprising from about 10 to about 75% by weight of: ##STR4## where R.sup.1 is selected from the group consisting of an alkyl with from 1 to 12 carbon atoms and an alkyl with from 1 to 12 carbon atoms substituted by aryl groups selected from the group consisting of phenyl, naphthyl, anthryl, and biphenyl and R.sup.2 is selected from the group consisting of methyl, ethyl, chloro, bromo and hydrogen. It was further disclosed that transport layers comprising the above material were found to have a high glass transition temperature (T.sub.g). It was also stated that the material retained its electrical properties after extensive cycling and exposure to the environment, i.e. oxygen, ultraviolet radiation, elevated temperatures, etc.
Belgian Pat. No. 753,415 discloses a photoconductive composition for use in electrophotographic elements. The photoconductive composition comprises substituted xylylidene of the general formula: ##STR5## wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 represent an alkyl or substituted alkyl group, an aryl or substituted aryl group,
R.sup.5 and R.sup.6 represent a hydrogen or hydroxy group, PA1 Ar represents a phenylene or substituted phenylene group, and PA1 R.sup.7 and R.sup.8 represent a substituted or unsubstituted alkyl group, a substituted or unsubstituted aryl group or hydrogen.
It is disclosed that "elements containing these photoconductors are markedly stable to oxidation and have good shelf life even at elevated temperatures compared to many other photoconductive compounds".
However, there is a need for electrophotographic elements which possess a high T.sub.g and at the same time are resistant to oxidation. High Tg is desirable, for example, when an element is used in a thermal transfer process comprising the direct thermal transfer of a toner image from a reusable electrophotographic element to a plain paper receiver. In such a process, toner is applied directly to the surface of the electrophotographic element, the receiver is positioned directly thereover and the sandwich is heated. It is necessary that the toner fully adhere to the receiver and then strip cleanly away from the element without damaging the element surface. This operation is achieved more readily if, despite the high temperature used, the element remains in a glassy state rather than transforming to a rubbery state, i.e., the element is operating below its T.sub.g. In addition it is important that the materials used in electrophotographic elements be resistant to oxidation and not form a dye derivative which causes undesirable coloration and/or affects spectral sensitivity.